Typical implantable cardiac stimulation devices such as pacemakers or implantable cardioverter defibrillators (ICDs) are capable of detecting arrhythmias and applying therapy to the heart. An arrhythmia is an abnormal heart beat pattern. One example of arrhythmia is bradycardia wherein the heart beats at an abnormally slow rate or wherein significant pauses occur between consecutive beats. Other examples of arrhythmias include tachyarrhythmias wherein the heart beats at an abnormally fast rate. In atrial tachycardia, the atria of the heart beat abnormally fast. In ventricular tachycardia, the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, a tachycardia is typically not fatal. However, some types of tachycardia, particularly ventricular tachycardia, can accelerate into ventricular fibrillation (VF). Ventricular fibrillation is an arrhythmia wherein the heart beats chaotically such that there is little or no net flow of blood from the heart to the brain and other organs. Ventricular fibrillation, if not terminated within minutes, is fatal. Hence, it is highly desirable to prevent or terminate arrhythmias, particularly arrhythmias of the type that can lead to ventricular fibrillation.
An arrhythmia is typically treated by delivering electrical shocks to the heart to restore a natural sinus rhythm in which the heart beats at a normal rate. Failure to promptly detect an arrhythmia (such as a low amplitude ventricular fibrillation) can result in a delay in the delivery of electrical shocks with a reduced likelihood of restoring the sinus rhythm. In this regard, it has been proposed to use the detection of loss of capture (LOC) of a series of ventricular pacing pulses as a means for detecting low amplitude VF and for triggering delivery of a high output defibrillation shock. See U.S. Pat. No. 5,350,401, by Levine, which is incorporated herein by reference. With that technique, upon detection of loss of capture of a ventricular pulse, the ventricular pulse output magnitude is increased and another pulse is delivered. If that pulse also fails to capture, the output magnitude is increased again. This process proceeds until either a ventricular pulse captures or until a maximum pulse output level is reached. If the maximum output is reached and the ventricular pulses still do not evoke capture, a determination is thereby made that a low amplitude VF may have occurred and a defibrillation shock may be delivered to terminate the VF. Although the technique is effective in eventually detecting low amplitude VF, the need to deliver a series of ventricular pulses with different pulse magnitudes delays the detection of VF, thus potentially reducing the effectiveness of subsequent shock therapy.
Thus, conventional arrhythmia detection techniques do not always detect arrhythmias as quickly as desired, resulting in a reduced likelihood that subsequent therapy will be successful. Conversely, conventional arrhythmia detection techniques may incorrectly detect arrhythmias, resulting in inappropriate therapy. In these situations, electrical shocks are delivered to the heart when no therapy is desired. Consequently, the patient may experience sensations such as pain from the electrical shocks when the patient's heart is functioning normally. In light of the above, there exists a need for improving the accuracy and reliability of heart arrhythmia detection.